Band broadening in SEC

Size-exclusion chromatography separation differs significantly fh)m other HPLC methods. For example, retention in SEC corresponds generally to values of k < 0. Similarly, there is no stationary phase apart from the stagnant mobile pha.se within the particle pores. Therefore there is no diffusion williin (he stalioniiry phase D, 0). This leads to equations for band broadening in SEC of a somewhat different form. 

Band broadening in SEC is simpler than for other HPLC methods, and as a result we can use SEC band width data to derive accurate values of the fundamental parameter p (Section V,C). For SEC, the Knox parameter B becomes [cf. Eqs. (36), (62)] ... 

A minimum sum of all three dispersion processes yields the "optimum velocity" at this velocity the column has maximum efficiency. The B term decreases with increasing flow rate and the C term increases with increasing flow rate. The A term is Independent on flow rate variation. Band broadening in SEC separation is mainly controlled by the mass transfer terms. The longitudinal effect (B term) is insignificant except for small molecules, and a minimum HETP is not usually observed in SEC. 

Along with retention, it is important to note that band broadening in FFF is also subject to theoretical analysis and subsequent application. As in SEC, band broadening is expressed in terms of plate height H. For most practical... 

While the fabrication of the optimum profiles described above requires the use of isotropic wet etching techniques, dry etching methods may also be used to produce geometries that minimize band broadening in pressure-driven flow systems. In this case, a shallow etch in the side-regions by an amount Ad = ad followed by a deep etch of the entire channel by a distance d yields a cross sec-... 

In order to achieve the best efficiency the SEC column should be operated at optimized operating parameters. The most important ones are flow rate [cf. van Deemter equation for band-broadening effects (21)], sample viscosity (depends on molar mass and concentration of the sample), and injection volume (7). 

D. Tong and K. D. Battle, Band broadening during mobile phase change in unified cliromtography (GC-SEC) , 7. Microcolumn Sep. 5 237-243 (1993). 

Th-FFF is also a good technique if polydispersities of very narrow samples from anionic polymerization need to be determined. This was demonstrated for polystyrene from band-broadening data in th-FFF with a higher accuracy than the results obtained by SEC on the same samples [75]. 

Instrumental band broadening or axial dispersion can cause calibration errors when employing polydisperse standards. Correction of the polydisperse standard calibration data for instrumental band broadening will minimize the effect on molecular weight analyses of polymer samples. However, as previously demonstrated in this report, when low dispersion SEC columns are employed instrumental band broadening is minimized and the effect on use of linear calibration methodology is negligible. 

The concentration effects for the oligomers and also for the excluded (high) polymer species, are usually small or even negligible, k values depend also on the thermodynamic quality of eluent [108] and the correlation was found between product A2M and k, where A2 is the second virial coefficient of the particular polymer-solvent system (Section 16.2.2) and M is the polymer molar mass [109]. Concentration effects may slightly contribute to the reduction of the band broadening effects in SEC the retention volumes for species with the higher molar masses are more reduced than those for the lower molar masses. 

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